A solar-powered Swiss airplane is back in the air as it attempts a 35,000-kilometer trip around the world without a drop of fuel. The Solar Impulse 2 flew out of central Japan early Monday after an unscheduled month-long stop.

Si2, piloted by Swiss pilot Andre Borschberg, one of the plane's creators, left Nagoya, Japan, for its trans-Pacific flight, the longest leg of the plane's journey.

If weather conditions over the Pacific do not remain favorable, the plane will be recalled. However, there will come a point-of-no-return in the crossing of the Pacific Ocean when the aircraft will not be able to make it back to Japan. The flight over the Pacific to Hawaii is risky because there is no place to land in an emergency.

The aircraft was not scheduled to land in Japan, but was diverted there at the beginning of June on its trip from Nanjing in eastern China to Hawaii because of bad weather.

Solar Impulse was re-scheduled to start its trans-Pacific sojourn last week, but the flight was canceled at the last minute because of bad weather conditions.

Si2 is the brainchild of two Swiss scientists, Bertrand Piccard and Borschberg. It took them 12 years to build the aircraft.

The carbon fiber single-seat Solar Impulse 2 has a 72-meter wingspan, which is longer than the wingspan of a Boeing 747, and weighs about as much as a car. The 17,000 solar cells built into the wings harness the sun's energy.

Piccard and Borschberg say they are not out to revolutionize the aviation industry, but are instead aiming to demonstrate that actual alternative energy sources and new technologies can achieve what some consider impossible.

The aircraft's route includes stops in Oman, India, Myanmar and China. After crossing the Pacific Ocean via Hawaii, the plane will make up to three stops in the U.S., touching down in Phoenix, Arizona and New York City, and possibly another location depending on weather conditions.

The final legs after crossing the Atlantic include a stopover in southern Europe or North Africa before arriving back in Abu Dhabi, where the journey started, in late July or early August.